1. Field of the Invention
This invention relates to the carbothermic production of aluminum from aluminum oxide and a carbon-containing material. It especially relates to purifying an aluminum reduction furnace product by removal of most of the relatively small amount of Al.sub.4 C.sub.3 therein. It specifically relates to such purification by reacting occluded aluminum carbide with aluminum oxide at extraction mode temperatures.
2. Description of the Prior Art
Reviewing the literature and the patent are readily indicates that there has been much activity by many people in an attempt to define adequately a thermal process which can compete advantageously with the conventional electrolytic methods of preparing aluminum. The art has long been aware of the many theoretical advantages which can flow from the use of a thermal reduction method for the production of aluminum as opposed to an electrolytic method. These advantages are becoming increasingly important as energy costs continue to increase. Unfortunately, the vast majority of such carbothermic processes have not resulted in a significant production of aluminum in a substantially pure state.
Specifically, these efforts have failed because they have invariably produced a mixture of aluminum metal and aluminum carbide. When such a mixture of 10-20% carbide or more cools to about 1400.degree. C., the aluminum carbide forms a cellular structure that entraps liquid aluminum; thus the mixture becomes difficult to pour. In consequence, unless extremely high temperatures are maintained throughout all of the steps, process manipulations of the mixture, in order to purify it, become extremely difficult, if not impossible.
The difficulty in producing aluminum with respect to thermal processes does not reside in the formation of the aluminum via reduction of the alumina-bearing ores, but rather, in the recovery of aluminum in a substantially pure state. The patent art, as well as the literature, is full of theories and explanations with respect to various back reactions which can take place between aluminum and the various carbon-containing compounds in the feed.
For example, U.S. Pat. No. 3,971,653 utilizes a slag containing an alumina mole fraction (N*=moles Al.sub.2 O.sub.3 /(moles Al.sub.2 O.sub.3 +moles Al.sub.4 C.sub.3) of 0.85 at a temperature of 2100.degree. C., with recycle of Al.sub.4 C.sub.3 -containing dross to the portion of the slag which is at reduction temperature. However, because the entire reaction to produce metal occurs at N*=0.85, the vaporization load is very high and the process power consumption is high.
U.S. Pat. Nos. 2,974,032 and 2,828,961 have described results that are typical of those to be expected from carbothermic reduction of a stoichiometric charge of alumina and carbon in a conventional electrically heated smelting furnace. The metal produced from the former process contains 20-37% Al.sub.4 C.sub.3 ; the metal produced by the latter process contains 20% Al.sub.4 C.sub.3. These processes are limited because reactive carbon and/or aluminum carbide is always present in contact with the metal that is produced and because time is available for the metal to react with the carbon and then to dissolve carbide up to its solubility limit.
One solution to the general problem of obtaining substantially pure aluminum from a carbothermic process is disclosed and claimed in U.S. Pat. No. 3,607,221. Although the process of this patent does result in the production of aluminum in a substantially pure state, extremely high operating temperatures are nevertheless involved which can lead to problems with respect to materials of construction. Another method for recovering substantially pure aluminum via a carbothermic process is disclosed and claimed in U.S. Pat. No. 3,929,456. The process of this patent also results in the production of substantially pure aluminum via a carbothermic process, but it does require careful control of the way the charge is heated in order to avoid aluminum carbide contamination.
By far, the most common technique disclosed in the prior art in attempting to produce aluminum of a high degree of purity has been directed to various methods of treating the furnace product which has conventionally contained about 20-35 weight percent of aluminum carbide. Thus, there are conventional techniques disclosed in the prior art, such as fluxing a furnace product with metal salts so as to diminish the amount of aluminum carbide contamination.
Unfortunately, the molten salts mix with the carbide so removed and it is costly to remove the carbide from the salts so that the carbide can be recycled to the furnace. Without such recycle, the power consumption and furnace size become uneconomical in comparison with prior methods practiced commercially for making aluminum.
U.S. Pat. No. 3,975,187 is directed towards a process for the treatment of carbothermically produced aluminum in order to reduce the aluminum carbide content thereof by treatment of the furnace product with a gas so as to prevent the formation of an aluminum-aluminum carbide matrix, whereby the aluminum carbide becomes readily separable from the aluminum. Although this process is very effective in preserving the energy already invested in making the aluminum carbide, it requires a recycle operation with attendant energy losses associated with material handling.
As disclosed in U.S. Pat. No. 4,099,959, a molten alumina slag is circulated through ducts, while being resistance heated in inverse relationship to the cross-sectional areas of the ducts, into alternating low and high temperature zones. The low-temperature zone is at a temperature high enough to produce aluminum carbide, and the high-temperature zone is at a temperature high enough to react aluminum carbide with alumina and produce aluminum. Off gases are first scrubbed through a first charge column containing only carbon and then through a second charge column containing only alumina in order to preheat these charge materials without forming a "sticky" charge because of partial melting of aluminum oxycarbide. The low and high temperature zones operate entirely within the molten range for a slag composition within N* values of 0.82-0.85.
U.S. Pat. Nos. 3,929,456 and 4,033,757 disclose methods for carbothermically producing aluminum containing less than 20% Al.sub.4 C.sub.3, i.e., 5-10%, which comprise striking an open arc intermittently to a portion of the surface of the charge to be reduced.
However, advances have now been made in the art, wherein aluminum that is contaminated with about 20% aluminum carbide can be treated so as to obtain aluminum of commercial purity. One such technique is described in U.S. Pat. No. 4,216,010. This technique is adaptable to the production of aluminum containing less than 20% Al.sub.4 C.sub.3 (i.e., 10%). It comprises the step of contacting a product containing from 20-35% Al.sub.4 C.sub.3 with a melt rich in alumina in the absence of reactive carbon. Such purification techniques can impart commercial vitality to older carbothermic processes producing heavily contaminated aluminum. Thus it becomes worthwhile to locate the best existing prior art and to improve the effectiveness thereof.
The process of U.S. Pat. No. 4,216,010 is directed particularly towards treatment of aluminum which is contaminated with from about 20 to about 35 weight percent of aluminum carbide, which is that amount of carbide contamination which is produced by a so-called conventional carbothermic reduction furnace, but it may also be used to treat aluminum which is contaminated with from about 2 to about 10 weight percent aluminum carbide as would be produced in furnaces used primarily for the production of aluminum such as those described in U.S. Pat. Nos. 3,607,221 and 3,929,456.
The novel process of U.S. Pat. No. 4,216,010, all of which is hereby incorporated herein by reference, is carried out simply by heating the furnace product contaminated with aluminum carbide with a molten slag containing substantial proportions of alumina so as to cause the alumina in the slag to react with the aluminum carbide in the furnace product, thereby diminishing the content of aluminum carbide in the furnace product. The expression "alumina in the slag to react with the aluminum carbide" is intended to describe various modes of reaction. While not wishing to be limited to a particular theory of operation, nevertheless, it appears that at least 2 modes of reaction as between the alumina in the slag and the aluminum carbide in the furnace product are possible.
One such mode can be described as the "reduction mode" and it involves reaction between the alumina in the slag and the aluminum carbide in the furnace product at reduction conditions so as to produce aluminum metal. One way of ascertaining operation in this mode is by measuring the evolution of carbon monoxide.
Another such mode of reaction can be described as the "extraction mode" and it involves reaction between the alumina in the slag and the aluminum carbide in the furnace product so as to produce non-metallic slag compounds such as aluminum tetraoxycarbide, as opposed to producing liquid aluminum. Such "extraction mode" reactions occur at temperatures insufficient to cause reduction to produce additional aluminum and can occur without causing the evolution of carbon monoxide.
In general, temperatures of at least 2050.degree. C. are necessary for the "reduction mode" operations at reaction zone pressures of one atmosphere. At any given pressure, the temperature required for "reduction mode" operation increases, as the level of aluminum carbide in the metal decreases. It is to be understood that "extraction mode" operations can take place below 2050.degree. C., but the "extraction mode" can also take place along with the "reduction mode".
As taught in U.S. Pat. No. 4,216,010, decarbonization furnaces are operated to reduce aluminum carbide content of the furnace product from the primary furnace by adding alumina to the slag layer (containing CaO as a melting point depressant) to maintain a composition equivalent to a weight ratio of alumina to aluminum carbide in the range of 80-97% alumina, balance aluminum carbide, and preferably in the range of 85-90% Al.sub.2 O.sub.3, balance Al.sub.4 C.sub.3, using heat from open arcs in the complete absence of reactive carbon. The added alumina forms a cover over the melted primary furnace product, and additional primary furnace product is added to the decarb furnace without disturbing the alumina cover. After the carbide in the furnace product has been reduced to 2% by the extraction mode of operation at about 2000.degree. C. in the decarb furnace, the metal layer is tapped to a holding or getter furnace where fluxing with Tri-Gas, according to the process of U.S. Pat. No. 3,975,187, for example, converts the metal to commercially pure aluminum.
However, the essence of this procedure is the formation of a solid alumina dome that is maintained over the alloy but is not in contact with it. This dome is necessarily sintered by the radiant heat from the arcs and provides thermal insulation of the melt and also recovers any aluminum vapor and aluminum oxide vapors that are evolved.
The presence of this structure requires a charging apparatus which can add liquid Al-Al.sub.4 C.sub.3 alloy and recycled slag to the furnace without disturbing the dome. Al.sub.4 C.sub.3 extraction takes place across the lower slag/metal interface, and the required alumina is added by controlled melting of the dome along its undersurface. This sintered alumina layer is stoked only when insufficient alumina is added by the melting process, and then the dome is immediately rebuilt. Therefore, solid alumina is never placed in contact with Al-Al.sub.4 C.sub.3 alloy except when alumina additions cannot be accomplished by the standard procedures. Even though alumina additions, by melting from the dome, must fall through the alloy and Al.sub.4 C.sub.3 extraction must take place while such drops are falling therethrough, the operating procedures of U.S. Pat. No. 4,216,010 do not optimize this phenomenon, and in fact, the formation of two-phase layers must generally be avoided in order to minimize slag entrainment in the metal product during tapping thereof.
In operation according to this procedure, the material throughout has been found to be limited by the slow rate of mass transfer which causes long holding times and increased energy consumption. To improve mass transfer, the concentration difference and/or the interfacial contact area must be increased. Because the ability to increase the concentration gradient is limited, it is necessary that the contact area be maximized.